739 research outputs found
Fidelity approach to the disordered quantum XY model
We study the random XY spin chain in a transverse field by analyzing the
susceptibility of the ground state fidelity, numerically evaluated through a
standard mapping of the model onto quasi-free fermions. It is found that the
fidelity susceptibility and its scaling properties provide useful information
about the phase diagram. In particular it is possible to determine the Ising
critical line and the Griffiths phase regions, in agreement with previous
analytical and numerical results.Comment: 4 pages, 3 figures; references adde
Heavy-to-light form factors: sum rules on the light cone and beyond
We report the first systematic analysis of the off-light-cone effects in sum
rules for heavy-to-light form factors. These effects are investigated in a
model based on scalar constituents, which allows a technically rather simple
analysis but has the essential features of the analogous QCD calculation. The
correlator relevant for the extraction of the heavy-to-light form factor is
calculated in two different ways: first, by adopting the full Bethe-Salpeter
amplitude of the light meson and, second, by performing the expansion of this
amplitude near the light cone . We demonstrate that the contributions to
the correlator from the light-cone term and the off-light-cone terms
have the same order in the expansion. The light-cone
correlator, corresponding to , is shown to systematically overestimate
the full correlator, the difference being , with
the continuum subtraction parameter of order 1 GeV. Numerically, this
difference is found to be 10-20%.Comment: revtex 14 pages, version to be published in Phys. Rev. D (discussion
in Sect. 3 extended, example in Sect. 4 added
Ni abundance in the core of the Perseus Cluster: an answer to the significance of resonant scattering
Using an XMM-Newton observation of the Perseus cluster we show that the
excess in the flux of the 7-8 keV line complex previously detected by ASCA and
BeppoSAX is due to an overabundance of Nickel rather than to an anomalously
high Fe He/Fe He ratio. This observational fact leads to the
main result that resonant scattering, which was assumed to be responsible for
the supposed anomalous Fe He/Fe He ratio, is no longer required.
The absence of resonant scattering points towards the presence of significant
gas motions (either turbulent or laminar) in the core of the Perseus cluster.Comment: 29 pages, 10 bw figures, accepted for publication in the
Astrophysical Journa
Bipartite quantum states and random complex networks
We introduce a mapping between graphs and pure quantum bipartite states and
show that the associated entanglement entropy conveys non-trivial information
about the structure of the graph. Our primary goal is to investigate the family
of random graphs known as complex networks. In the case of classical random
graphs we derive an analytic expression for the averaged entanglement entropy
while for general complex networks we rely on numerics. For large
number of nodes we find a scaling where both
the prefactor and the sub-leading O(1) term are a characteristic of
the different classes of complex networks. In particular, encodes
topological features of the graphs and is named network topological entropy.
Our results suggest that quantum entanglement may provide a powerful tool in
the analysis of large complex networks with non-trivial topological properties.Comment: 4 pages, 3 figure
Thermodynamic formalism for dissipative quantum walks
We consider the dynamical properties of dissipative continuous-time quantum
walks on directed graphs. Using a large-deviation approach we construct a
thermodynamic formalism allowing us to define a dynamical order parameter, and
to identify transitions between dynamical regimes. For a particular class of
dissipative quantum walks we propose a quantum generalization of the the
classical PageRank vector, used to rank the importance of nodes in a directed
graph. We also provide an example where one can characterize the dynamical
transition from an effective classical random walk to a dissipative quantum
walk as a thermodynamic crossover between distinct dynamical regimes.Comment: 8 page
Simulation of Diffusion Time of Small Molecules in Protein Crystals
SummaryA simple model for evaluation of diffusion times of small molecule into protein crystals has been developed, which takes into account the physical and chemical properties both of protein crystal and the diffusing molecules. The model also includes consideration of binding and the binding affinity of a ligand to the protein. The model has been validated by simulation of experimental set-ups of several examples found in the literature. These experiments cover a wide range of situations: from small to relatively large diffusing molecules, crystals having low, medium, or high protein density, and different size. The reproduced experiments include ligand exchange in protein crystals by soaking techniques. Despite the simplifying assumptions of the model, theoretical and experimental data are in agreement with available data, with experimental diffusion times ranging from a few seconds to several hours. The method has been used successfully for planning intermediate cryotrapping experiments in maltodextrin phosphorylase crystals
Adiabatic quantum algorithm for search engine ranking
We propose an adiabatic quantum algorithm for generating a quantum pure state
encoding of the PageRank vector, the most widely used tool in ranking the
relative importance of internet pages. We present extensive numerical
simulations which provide evidence that this algorithm can prepare the quantum
PageRank state in a time which, on average, scales polylogarithmically in the
number of webpages. We argue that the main topological feature of the
underlying web graph allowing for such a scaling is the out-degree
distribution. The top ranked entries of the quantum PageRank state
can then be estimated with a polynomial quantum speedup. Moreover, the quantum
PageRank state can be used in "q-sampling" protocols for testing properties of
distributions, which require exponentially fewer measurements than all
classical schemes designed for the same task. This can be used to decide
whether to run a classical update of the PageRank.Comment: 7 pages, 5 figures; closer to published versio
Spin networks, quantum automata and link invariants
The spin network simulator model represents a bridge between (generalized)
circuit schemes for standard quantum computation and approaches based on
notions from Topological Quantum Field Theories (TQFT). More precisely, when
working with purely discrete unitary gates, the simulator is naturally modelled
as families of quantum automata which in turn represent discrete versions of
topological quantum computation models. Such a quantum combinatorial scheme,
which essentially encodes SU(2) Racah--Wigner algebra and its braided
counterpart, is particularly suitable to address problems in topology and group
theory and we discuss here a finite states--quantum automaton able to accept
the language of braid group in view of applications to the problem of
estimating link polynomials in Chern--Simons field theory.Comment: LateX,19 pages; to appear in the Proc. of "Constrained Dynamics and
Quantum Gravity (QG05), Cala Gonone (Italy) September 12-16 200
Decay constants of heavy pseudoscalar mesons from QCD sum rules
We revisit the sum-rule extraction of the decay constants of the D, Ds, B,
and Bs mesons from the two-point correlator of heavy-light pseudoscalar
currents. We use the operator product expansion for this correlator expressed
in terms of the MSbar heavy-quark mass, for which the perturbative expansion
exhibits a reasonable convergence. Our main emphasis is laid on the control
over the uncertainties in the decay constants, related both to the input QCD
parameters and to the limited accuracy of the method of sum-rules. This becomes
possible due to the application of our procedure of extracting hadron
observables that involves as novel feature dual thresholds depending on the
Borel parameter. For charmed mesons, we find the decay constants f_D=206.2\pm
7.3(OPE)\pm 5.1(syst) MeV and f_Ds=245.3\pm 15.7(OPE)\pm 4.5(syst) MeV. For
beauty mesons, the decay constants turn out to be extremely sensitive to the
precise value of mb(mb). By requiring our sum-rule estimate to match the
average of the lattice results for f_B, a very accurate value mb(mb)=4.245\pm
0.025 GeV is extracted, leading to f_B=193.4\pm 12.3(OPE)\pm 4.3(syst) MeV and
f_Bs=232.5\pm 18.6(OPE)\pm 2.4(syst) MeV.Comment: 12 page
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